Reply To: Motors and thermal protection

Joost Borst

I am a bit rusty on motor theory but ill give it a go, learned so much here its the least I can do. 🙂

I’m not really familiar on how its done in the USA but a basic 1 phase motor has a capacitor in it to create a phaseshift.

If you look at the schematic it has the two main wires comming into the motor connected to a coil. There is another connection to a capacitor that feed into another coil that is physiclly at a different angle around the rotor (moving part). What bassicly happens is that the capacitor will delay the current going to the second coil by storing it. A capacitor is like a bucket with a hole in it.

So if you look at my really bad paint drawing 😉 you get your normal coil in the green and than the second coil in red.
There are a type of magnet in the rotor that will try to allign it self with the magnetic field created by the coils causing it to rotate. Since one of the coils is shifted in time the magnetic field will shift position causing the rotor to keep moving. Kinda like a dog chasing its tail.

If the capacitor is broken it cant make the phase shift so there isnt any delay, no delay means no rotation.
The motor will still draw power but wont rotate, the power has to go somewhere so it will turn in to heat. The fact that normally there is a cooling fan attached to the rotor it will overheat even quicker cause the fan isnt spinning.

I’m gonna agree with the others that probably the cheapest and most logical fix is gonna be to change the capacitor. Try finding someone that has a good knowledge of electric motors to help you out. Maybe ask him to measure the windings of the motor while your at it, isolation does degrade over the years and can cause shorts in the motor.

If your still wondering about the thermal protection, motors these days are usually equipt with Clixons or thermistors. A clixon is basicly a bi-metal switch that will switch once the motor is to hot and resets it self once its cooled of again. An thermistor is a heatsensitve resistor that can be used to measure the temperatuur more accurate.

Another cause of the trips can be a motor protection device (dont know the propper term in english) like a 3RV20 from Siemens. These device are like the circuit breakers in your house but they can be set to the specific motor current. They also have a pretty ingenius mechanical solution to make sure current is being drawn equally from all phases. I do doubt that that thing is in such an “experienced” machine.

Just to make it clear, if you dont know what your doing, dont mess with electricity it can hurt/kill you.